Polyethylene film is widely used in many packaging applications.
The type of polyethylene that was first commercially available is prepared under high pressure using a free radical initiator. This type of polyethylene is widely known as high pressure/low density polyethylene and as “LD” polyethylene. LD polyethylene is easy to process but monolayer films prepared with LD polyethylene have comparatively poor mechanical properties.
The use of Ziegler Natta (“Z/N”) catalysts enables the production of ethylene copolymers having better mechanical properties than LD polyethylene—but these copolymers are more difficult to process than LD polymers.
More recently, the use of metallocene catalysts has allowed the production of ethylene copolymers having enhanced impact strength (but, again, with the expense of poor processability in comparison to LD polyethylene).
The use of blends of polyethylenes to prepare monolayer films is well known. For example, it is known to blend LD polyethylene with Z/N catalyzed polyethylene or metallocene catalyzed polyethylene to improve processability—but the mechanical properties of the blends are compromised. It is also known to improve multilayer films using these polyethylenes. In practical terms, the loss of impact strength is especially problematic when blending LD polyethylene with a metallocene catalyzed polyethylene having a density of from about 0.915 to 0.930 g/cc. This is because lower density metallocene polyethylenes have such high impact strengths that losses are more easily tolerated. Because higher density metallocene resins have lower impact strength to begin with they are generally not used in film applications where impact strength is important.